A so-called Gilbert cell or four-quadrant multiplier is a cross-coupled differential amplifier having a gain that can be linearly controlled by modulating emitter bias current. The amplitude of the differential input RF signal can be linearly controlled by a differential AC voltage. Gilbert cells are commonly used in a number of applications, including mixers, automatic gain control (AGC) amplifiers, amplitude and sideband modulators, amplitude modulation (μM) and sideband detectors, frequency doublers and dividers, squaring and square-root circuits.
The typical implementation of a Gilbert cell mixer utilizes a simple differential pair as the input transconductance stage. This is cascode connected with the Gilbert switching quad, which is typically driven by a local oscillator signal. Such mixer designs suffer from a number of problems, including poor linearity of the transconductance differential amplifier input stage. Conventional techniques such as emitter degeneration can be utilized to improve linearity, but this has a direct, negative impact on noise performance of Gilbert cell mixers.
There is a need, therefore, for improved Gilbert cell mixer designs.